This invention pertains to amplifier circuits with low distortion, and more specifically BTL (Balanced Transformerless) amplifier circuits with low distortion in which the principle of a feed forward technique is incorporated.
Among those known amplifier circuits, BTL amplifier circuits are extensively used particularly in the field of such as audio equipments due to certain inherent advantages over other conventional amplifier circuits such as SEPP (Single Ended Push-pull) amplifier circuits. A BTL amplifier circuit, when compared with a SEPP amplifier circuit, has an advantage that the former can obtain, with the same power source voltage as applied to the latter, about two times more load current and four times more output power than those of the SEPP amplifier circuit.
The principle of BTL amplifier circuits can be better understood by referring to FIG. 1. FIG. 1 shows a block diagram of one example of a prior BTL amplifier circuit comprising two amplifiers, designated by reference numerals 1 and 2 respectively and having gains which are equal in absolute value and opposite in phase to each offer. An input signal is applied via an input terminal T.sub.1 to the amplifiers 1 and 2. The amplifier 1 effects amplification of the input signal and develops at an output terminal T.sub.2 an output signal same in phase as the input signal, while the amplifier 2 amplifiers the input signal and develops at an output terminal T.sub.3 an output signal opposite in phase to the input signal. A load, e.g. a loudspeaker L, connected between the terminals T.sub.2 and T.sub.3 is driven by the two output signals each having the same amplitude and opposite polarity to each other.
It should be noted here that distortions generated in the respective amplifiers 1 and 2 are mostly opposite in phase to each other as is similar in the case of the output signals appearing at the output terminals T.sub.2 and T.sub.3. The distortions are accordingly superposed additively at the load. This is particularly true when the amplifiers 1 and 2 have substantially the same characteristics and hence the generated distortions within the amplifiers 1 and 2 have substantially the same amplitude and are opposite in phase to each other. In such a case the addition of the distortions across the load is enhanced to some degree.
In order to decrease the distortions of the prior BTL power amplifier circuit, therefore, it has been requisite to use the amplifiers 1 and 2 both with a low distortion factor, for example, class-A amplifiers. In result, this had led to a high cost of the circuit arrangement and moreover such disadvantage has been brought that a decrease of the power efficiency occurs in the amplifier circuits.